Our long term goal is to learn developmental mechanisms that underlie the formation of specific neurons and their connections in the central nervous system of a vertebrate. Our studies are focused on the earliest neurons that arise and form a functional network in the embryo of the zebrafish Brachydanio rerio. We propose to examine a hypothesis that growth of an identified dendrite, the Mauthner neuron lateral dendrite, is in part shaped by developmental interactions with sensory axons that synapse with the dendrite. We will use specific labeling procedures and both light and electron microscopy to learn the spatiotemporal sequence of development of contacts by specific sensory growth cones, and how this sequence is related to dendritic outgrowth. We will also test for a developmental role of the specific inputs, by using a laser to ablate the sensory neurons whose axons normally are the first to contact the Mauthner cell. Later we will examine the morphology of such deprived Mauthner cells and their synapses with the remaining types of sensory neurons. In parallel experiments we will use mutations to identify and characterize genes important for the development of the Mauthner cell and the neurons to which it is synaptically connected. We will mutagenize zebrafish from lethal-free strains, and identify mutations of interest by rapid nondestructive assays of specific function and morphology of the Mauthner neuron in parthenogenetic embryos. We will characterize the mutations genetically, and use mutant embryos to analyze the role these genes play in neuronal development and functioning.